the polish for the hearts is the dhikr (remembrance) of Allah:

Allah's Messenger (sallAllahu 'alayhi wasallam) said •► "For
everything there is a polish, and the polish for the hearts is the
dhikr (remembrance) of Allah. There is nothing more potent in saving a
person from the punishment of Allaah than the dhikr of Allah." It was
said: 'Not even Jihaad in the path of Allah.' So he replied•►"Not even
if you were to continue striking with yoursword until it breaks."
[Saheeh: Related by Ahmad(4/ 352), from Mu'aadh ibn Jabal radiallaahu
'anhu. It was authenticated by al-Albaanee in Saheehul-Jaami'
(no.5644)]

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